client-rust: Make Histogram::observe calls atomic across collects

[Max Inden]

Hi everyone,

With Prometheus v2.17.0 queries can only see full scrapes, but they
could still access inconsistent data if the scrape itself is inconsistent.

This needs to be prevented at the target side. Thanks to Björn the
Prometheus Golang client prevents this for histograms by introducing the
notion of a hot and a cold shard preventing collect and observe calls to
interleave [1].

I have ported this mechanism to the Prometheus Rust client. Given that
it is a rather involved patch I would very much appreciate any
additional reviewers. The more eyes the better.

https://github.com/tikv/rust-prometheus/pull/314

Thanks for the help.
Max

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[1] https://github.com/prometheus/client_golang/pull/457